1. Field Of The Invention
This invention relates to an engine component assembly, and more particularly to wrist pins for deployment therein.
2. Related Art Statement
The implications of mass reduction in reciprocating components of piston engines is well defined. Two strategies are conventionally utilized to take advantage of reduced mass: power may be increased by a factor proportional to the mass reduction by allowing the dynamic loads to remain constant while increasing the engine speed (if valve train components are upgraded simultaneously); or fuel economy benefits may be realized by reducing mass and associated friction, while maintaining a constant engine speed.
Numerous reciprocating piston engines are known which utilize a piston pivotally connected to a connecting rod by a wrist pin. In use, the piston forms a movable gas-tight plug which confines a charge within a cylinder and transmits to the connecting rod forces generated by combustion of the charge. Since the piston forms a guide and a bearing for a piston-connecting end of the connecting rod, the piston operates under lateral thrust due to an oblique inclination of that rod. Wrist pin bosses are formed within the skirt located on the underside of the piston. If the wrist pin is fixed in relation to the connecting rod, and the wrist pin pivots freely within holes defined in wrist pin bosses, the lateral thrust loading tends to be distributed over a large bearing area.
Ideally, the combined weight of the piston and wrist pin is kept to a minimum to reduce vibration and the inertial loading on related bearings, and to avoid friction and other losses entailed in accelerating the piston/wrist pin in both directions.
Design considerations for piston/wrist pins include anticipating and accommodating considerable variation in operating temperatures--from starting cold, to very high temperatures at maximum output. Superimposed upon these variations are those which may be encountered in any given geographic location.
Thermal flow in a piston is from a crown, then outwardly out to a ring belt, whence heat is transferred through rings to cylinder walls (and thence to a coolant). The wrist pin is subjected to those thermal variations by heat transferred down to the skirt and across the bearing surfaces.
In modern internal combustion engines, if the height of the piston is reduced, the wrist pin becomes deployed more closely to the hot crown. As a result, lubricating difficulties are experienced due to the higher loads imposed by increased engine performance and higher operating temperatures.
Virtually all pistons are tapered from top to bottom. This compensates for differing expansion due to thermal gradients and variations in thickness of metal needed to transmit the gas and inertial loading from the crown to wrist pin bosses.
The wrist pin may be one of the most highly stressed engine components. Wrist pins must withstand bending, compression, and shear stresses without significant deformation over time in varying conditions of loading and temperature. If the wrist pin is unable to resist significant bending forces, the fatigue behavior of the associated wrist pin bosses will be adversely affected. If sub-optimal materials or dimensions are selected, partial seizure or scuffing may occur in the wrist pin bosses due to the very small clearances which are generally provided.
Where lighter pistons are used, the wrist pin contributes to a higher proportion of the total mass of the piston and wrist pin. In several cases, the wrist pin may account for 30%-40% of the total piston-wrist pin mass.
If the wrist pin is formed from a substantial mass of a suitable metallic material, it will generally not fail catastrophically, since deformation is likely to be limited to the elastic range. Nevertheless, the search resumes for suitable materials that may provide an increase in stiffness of the wrist pin.
Although many wrist pins in use today are made of case-hardened steel, some have been developed which are entirely formed from a ceramic material. Such approaches tend to suffer from the inability of present designs to resist failure of the pin and contain fracture debris to avoid catastrophic failure of associated engine components. Adverse damping characteristics of an all-ceramic wrist pin also adversely impact the noise associated with an oscillated piston/pin/connecting rod assembly. Additionally, to minimize friction, the outside diameter of ceramic piston pins need to be ground to achieve a desired surface texture. Accordingly, it would be desirable to provide a design wherein the outside diameter of a metal shell is in stationary contact with adjacent ceramic material and in rotary contact with associated wrist pin bosses, thereby avoiding the need for potentially expensive post-forming operations, such as grinding.